In a meeting with Hiroshi Ogawa, Director for Infinite Undiscovery, and Hajime Kojima, Producer for the title, IGN learned for a second time at TGS this year that a game coming to Xbox 360 would require multiple discs. This time around, it's Infinite Undiscovery. The two game makers couldn't say exactly how many discs the game would require as it is still in the middle of production, but they were certain that it wouldn't fit on a single DVD. They also weren't sure how long the game would last, but estimated 30-40 hours.
Get used to the notion of a multiple disc games on Xbox 360. It's looking like you'll be seeing a lot of it in the future.